Visibility — Page 274, Lesson 257

I want to walk you through the different types of fog that affect aviation visibility, starting with radiation fog. Let's begin with radiation fog. This fog forms as a direct result of the Earth's surface losing heat at night through radiation. As the ground cools, it conductively cools the air immediately above it. A natural consequence of this radiative cooling at the surface is the development of a temperature inversion above the fog layer — and that inversion usually sits within the friction layer, which is the layer of the atmosphere where surface friction affects wind flow. Now, when does radiation fog occur? Predominantly in autumn and winter. It forms at night and in the early morning, because the lowest temperatures of the day occur just before dawn. There's an important timing detail here: the latest time at which radiation fog can still form is about 30 minutes after sunrise. Why that cutoff? Because after sunrise, the first insolation — that's incoming solar radiation — begins to heat the ground, creating thermal turbulence that can break up the fog. Where does radiation fog form? Over land, not over sea. The reason is that over the sea there is very little diurnal variation — DV — of temperature. Diurnal variation means the difference between day and night temperatures. Over land that variation is large, so the ground can cool enough at night to produce fog. Within the land, radiation fog forms first in valleys because of the katabatic effect — that's the downslope flow of cold, dense air that drains into low-lying areas. The weather patterns that favour radiation fog are anticyclones, ridges, and cols — these are all high-pressure or neutral pressure features with light winds and clear skies that allow maximum radiative cooling. How does radiation fog disperse? Two main ways. First, by insolation — solar heating — which causes convection that lifts the fog and also helps evaporate the lower layers. Second, by a strong wind that lifts the fog to form stratus cloud — so the fog doesn't disappear, it just rises off the surface and becomes a low cloud layer. Here's a practical note for the UK: radiation fog usually clears by 1000 to 1100 hours local time, but it may persist all day in valleys and during winter months. Now let's move to hill fog, also called orographic fog. Hill fog is usually stratiform cloud — either stratus, abbreviated ST, or stratocumulus, abbreviated SC — whose base is lower than the summit of the hills. So from the pilot's perspective, the hilltops are obscured by cloud even though the cloud base may be well above the surrounding low ground. Hill fog can be generated in two ways: when moist stable air is forced to rise over the hills, forming what's called a cap cloud, or by normal turbulence action producing stratus and stratocumulus. Finally, advection fog. Advection fog is formed by the movement of warm, moist air over a cold surface. The key difference from radiation fog is that the air itself is moving — it's being advected, or carried horizontally. The cold surface can be either land or sea. So unlike radiation fog, advection fog can form over the sea as well as over land.